Creating spatial representations of the world for use in decision-making, navigation and many other applications, has grown into an enormous GIS (Geographical Information System, or sometimes, Global Information System) that links natural and man-made features with their relative and unique 3-D positions on or near Earth. Data include Earth topography and vegetation, as mostly gathered by satellite imagery. Other Features, such as the centerline of a road, are gathered by driving a vehicle with a GPS (Global Positioning System) system and noting the location of intersections and waypoints. Utility locations are input by surveying with GPS. Collected into GIS databases, the data subsequently are used for vehicle navigation, building operations, emergency response, environmental health, and a wide variety of other applications.
GIS data are often physical phenomena. Originally, GIS consisted of databases of satellite images of Earth, typically taken in multiple spectra. By comparing the various images over time, objects, changes, and patterns could be identified. While some features and their attributes can be imaged from satellites very efficiently, many physical objects are out of satellite view. Even where features are in view, it can be difficult to distinguish those features in a satellite view, and it is especially difficult to distinguish them automatically using software image analysis, for example. As a result, many features must be located manually, or local sensors must be placed at known locations to locate and/or track specific features. These methods can be expensive and time consuming, so most GIS data tends to be acquired once, and is updated rarely, if ever.
GIS data updates, however, are need for differential analyses in a number of different areas. In a security application, for example, change analysis may detect and locate intruders or re-locate moveable assets. For hazard detection, changing levels of water, heat, smoke, radiation, or gases may initiate investigations. For foresters, tree trunk sizes may indicate time to harvest.